1. Field of the Invention
The present invention relates to manometers and more particularly to manometers provided with improved air flow across a manometer assembly within a measurement chamber.
2. Description of the Prior Art
The dual cistern manometer was developed to avoid the meniscus vagaries attendant with mercury in glass tubes. For example, see U.S. Pat. No. 3,225,599 issued Dec. 28, 1965 to L. M. Schwien for a Manometer. Approximately 1,200 of such dual cistern manometers have been built. In the early 1980's, it was discovered that particularly in the measurement of low pressures, the mercury line contained in a stainless steel bellows hose was lying on a metal base which was often at an appreciably different temperature. Secondary errors were also encountered due to the use of a transparent front of the casing.
As the vertical height of manometers has been increased to achieve greater accuracy, the change in the temperature of the manometer along such height, or temperature gradient, has introduced new inaccuracies. To reduce temperature gradient-induced inaccuracies, it has been suggested that the temperature gradient itself be minimized. While some causes of temperature gradient have been eliminated, air-related causes of temperature gradient are more difficult to eliminate.
It has been suggested that a rectangular, outer, highly insulated shell be spaced from a rectangular, inner, measurement chamber, to provide an air curtain which recirculates the air from the top to the bottom of the inner rectangular chamber on all four sides. The air is drawn over an electrical resistance heater within a squirrel cage blower which is rotated about a vertical axis at the top of the inner chamber by an external motor. The air is heated by a thermal control system governed by the temperature of a flexible mercury line connecting vertically spaced cisterns that contain mercury. The temperature probe is at the mid-point of the mercury line. In tests of this structure, it has been found that the rectangular shell and the walls of the rectangular inner chamber resonate and interfere with the accuracy of the manometer reading. Isolation of the squirrel cage blower from the shell and chamber indicate that such resonance is not solely due to mechanical vibration within the squirrel cage blower.
Also, as the vertical height of manometers has been increased, it has been difficult to keep a column straight and vertical in the measurement chamber. As disclosed in U.S. Pat. No. 3,225,599, one such column and an adjacent threaded lead screw have cooperated to guide and adjust the vertical location of a movable cistern. Ideally, the column is straight and vertical so that the free surface of the mercury inside the movable cistern is horizontal. When the column is bowed or not vertical, a collar that rides along the column becomes tipped such that the movable cistern mounted on the collar is not level. With the movable cistern out of level, the free surface of the mercury is not horizontal, which interferes with the accurate operation of the manometer.